RF power amplifiers are key components in any high-frequency system and application. The demand for higher bandwidth and higher speeds for multimedia applications puts stringent requirements on these amplifiers such as high output power, linearity and efficiency. Amplifiers comprising bipolar transistors (such as heterojunction bipolar transistors (HBTs) are often used in preference to CMOS-based devices for RF power amplifiers because of their excellent performance at high frequencies.
A limitation of silicon based devices (e.g. SiGe HBTs) is that the smaller bandgap of silicon compared to some III-V materials typically gives rise to a lower breakdown voltage (BV), which can inhibit the operation of such devices at high voltages.
Efforts have been made to create higher-breakdown SiGe HBTs by optimization of collector profiles (trading off the breakdown voltage (BV) and cut-off frequency (fT)). For example, see:
D. R. Greenberg et al. “Large-signal performance of high-BVCEO graded epi-base SiGe HBTs at wireless frequencies ” IEDM Proceedings, pp. 32.3.1-32.3.4, 1997;
E. J. Preisler et al. “Integration of a 5.5V BVCEO SiGe HBT within a 200 GHz SiGe BiCMOS flow”, BCTM Proceedings, pp. 202-205, 2007;
B. Geynet et al., “High-voltage HBTs compatible with high-speed SiGe BiCMOS technology”, SiRF Proceedings, pp. 210-213, 2008; and
H. Mertens et al. “Extended high voltage HBTs in high-performance BiCMOS process”, BCTM Proceedings, 2011.
Other efforts have involved using reduced surface field (RESURF) effects (using either a field plate or pn junction) to improve performance in terms of BV×fT (see, for example, J. Melai et al., “A new sub-micron 24 V SiGe:C RESURF HBT”, ISPSD Conference, 2004).
R. Sorge et al., “Concept of vertical bipolar transistor with lateral drift region, applied to high voltage SiGe HBT”, SiRF, 2012 describes a vertical bipolar transistors having an additional lateral drift region introduced between a sub collector and collector contact region. This is described as enabling the fabrication of high voltage bipolar transistors for RF power applications. The introduction of an additional lateral drift region with a length of 1.2 μm increased the open base breakdown voltage BVCEO of the HBT 7V to 18V. The same device concept has been again shown to be able to have a much larger BVCEO, up to 35V, in R. Sorge et al., “Integration of a 50 BVCEO SiGe:C HBT into a 0.25 μm SiGe:C BiCMOS platform”, SiRF, 2014. However, one of the main bottlenecks of these devices is a high collector resistance. This can turn on the parasitic PNP of the device and cause a strong reduction in fT.
A device having a lateral drift region, which does not suffer from an increase in collector resistance would be especially promising for power amplifier applications.